The present invention is based on a device for draining flexible fluid containers, in particular blood bags in a single needle arrangement type extracorporeal system as is disclosed in U.S. Pat. No. 4,991,743, and is particularly suited to draining blood bags which form a part of a single needle arrangement-type extracorporeal system.
The single needle technology has long been a standard component in the procedure used in the preservative treatment of blood. Central to this procedure is the withdrawal and reinfusion of the unpurified or purified blood through a single needle, i.e. at a single point of penetration. This procedure is gentle on the containers and is less painful to the patient.
An important requisite for efficient dialysis treatment using the single needle method is the rapid withdrawal and reinfusion of blood so as not to increase inordinately the overall length of treatment.
In dialysis treatment using the single needle method, an important time determining factor is seen in the reinfusion of the purified blood. Normally, the purified blood is collected in a flexible container, then drawn from that container, once the arterial portion of the circulatory system is shut off, and injected through the needle back into the patient's blood circulation. The fluid must flow from the container as rapidly and as evenly as possible.
As a further requisite for efficient dialysis treatment, the pressure throughout the dialysis system must be as consistent as possible. This enables the quantity of treated blood, for example, to be precisely determined, allowing the proper amounts of medication to be added and pump capacities to be adjusted accordingly.
Known devices utilized for this purpose may be ascertained, e.g., from the embodiments described below.
In the aforementioned U.S. Pat. No. 4,991,473 a device is claimed which is capable of controlling the pressure applied to a flexible fluid container. This involves, specifically, a plate to which pressure is applied by means of scissor jacks, and which then transfers this pressure throughout the entire withdrawal phase to the surface of the flexible fluid container. However, this type of pressure control fails to account for pressure variations within the flexible fluid container during fluid withdrawal; nowhere does the referenced patent make reference to the fact that the pressure is adjusted to variations in pressure and volume.
German patent publication DE 39 11 587 A1 discloses a device in which a bag is disposed between two pressure plates, one of which is movable and capable of being actuated. This device is disadvantageous in that it requires a complex pressure monitoring device to be mounted to the pressure plates, necessitating close and lengthy supervision.
The two aforementioned references show that it was heretofore impossible to prevent via simple means undesirable pressure variations that consistently occur as blood is withdrawn from a flexible container.
Thus, reinfusion of purified blood using the single needle technique often produces unequal pressures within the dialysis system which lead to treatment durations that are difficult to monitor. Moreover, the length of treatment increases inordinately because, as the remainder of blood drains from the flexible container, the amount of blood transported per unit time decreases continually due to a decrease in pressure.
The problems surrounding pressure drops are clearly illustrated in FIG. 5. The device as claimed in U.S. Pat. No. 4,991,743, for example, exhibits properties characteristic of a semi-weighted pressure plate. With this device the drop in pressure, relative to the initial flow pressure, reaches ca. 50% by the time the remainder of the blood is being drained from the flexible container, due to the fact that the unchanging force is dispersed over an increasingly larger surface area of the bag which abuts the pressure plates, and with that a drop in pressure (force/surface) results. Under these conditions significant pressure fluctuations within the dialysis system and prolongation of the period required for reinfusion of blood into the patient (decreasing flow rate is a function of decreasing pressure) are unavoidable.